The Global Positioning System (GPS) is a location system that uses signals transmitted by satellites orbiting the earth. Because the GPS satellites provide accurate positioning 24 hours a day anywhere on earth, GPS technology has gained widespread use over the last decade in commercial and military applications. GPS receivers capable of determining geographic position are implemented in cell phones, laptop computers, PDA devices, automobiles and other vehicles, aircraft, and as stand-alone portable devices that may be worn or carried by a person or animal.
A GPS receiver determines its own position based on the signals transmitted by a select number of satellites that the GPS receiver is capable of receiving. The orbits and locations of GPS satellites are known in advance. This satellite position information is stored in a first data record known as the “almanac”. Each GPS satellite continually broadcasts the almanac information in a standard message to GPS receivers on earth. Each GPS receiver automatically collects and stores the almanac information from each satellite that the GPS receiver is able to detect.
However, the orbits of GPS satellites do not perfectly follow the almanac information. Ground-based radar stations monitor the naturally-occurring deviations between the actual position of each GPS satellite and the predicted information in the almanac. This deviation information is stored in a second data record known as the ephemeris. The ephemeris errors for a GPS satellite are transmitted to that GPS satellite, which in turn broadcasts the ephemeris errors as part of the standard message to GPS receivers on earth. Almanac information is useful for about one year before it must be updated. Ephemeris data is useful for about two hours before it must be updated.
Each GPS receiver uses the almanac information and the ephemeris data to determine very precisely the position of a GPS satellite at a given point in time. Once the positions of several GPS satellites are precisely known, a GPS receiver is able to determine the distance to each of the satellites according to the amount of time it takes the GPS signal to travel from the satellite to the GPS receiver. The GPS receiver then uses the distance information for several satellites to determine very precisely the position of the GPS receiver on earth. The GPS receiver information may include latitude information, longitude information, and altitude information (i.e., height above sea level).
The foregoing description is characteristic of a GPS receiver that operates in an autonomous mode. An autonomous GPS receiver determines its position entirely on its own, using only the signals from the GPS satellites. For the purposes of this disclosure, the autonomous mode of operation may also be referred to as “device autonomous fix (DAF)”. However, some GPS receivers operate in an assisted GPS (aGPS) mode. In assisted GPS mode, a GPS receiver may receive almanac and ephemeris data, and even position information, from a terrestrial (i.e., ground-based) network with which the GPS receiver may communicate by means of a wireless connection or a wireline connection. For example, a cell phone or wireless PDA containing a GPS receiver may communicate with a public or private wide-area network (WAN) using the CDMA2000 protocol, the GSM protocol, or the IEEE-802.11 protocol (or a similar Wi-Fi protocol.
There are two broad categories under which an assisted GPS receiver may fall. In a first category, a terrestrial network delivers GPS correlation parameters to the measurement engine (ME) of the GPS receiver. The measurement engine uses the correlation parameters to direct the correlators of the GPS receiver to specific search regions for the expected time and frequency offsets for each visible satellite. The measured time and frequency offsets are often called pseudo-ranges (PRs) and pseudo-Dopplers (PDs). Frequently, the set of measured correlation parameters are collectively referred to as pseudo-ranges, even though these parameters include both frequency offset measurements and other parameters.
However, the correlation search parameters may become stale very quickly, within 5 minutes or so. To overcome this problem, first and second order correction factors may be sent along with the correlation search parameters in order to allow the GPS receiver to correct for differences between the time at which the correlation data was created and the time at which the correlation data will be used. However, in this first category of assisted GPS mode, the actual calculation of true range, Doppler, time, and accurate position information is completed at a network server with receives from the GPS receiver the approximate location of the GPS receiver, its measured pseudo-range (PR) and pseudo-Doppler (PD) information, and the apparent GPS time at which the pseudo-range (PR) and pseudo-Doppler (PD) information was captured.
Thus, in this first category of aGPS devices, the terrestrial network sends GPS correlation parameters to the measurement engine (ME) of the GPS receiver. The measurement engine of the GPS receiver then sends approximate location, measured pseudo-range and pseudo-Doppler information, and apparent GPS time to the terrestrial network. The server in the terrestrial network uses the information received from the GPS receiver to calculate true range, Doppler, time, and position information and returns this information to the GPS receiver. For the purposes of this disclosure, this first category of assisted GPS mode of operation may also be referred to as “server-computed assisted fix (SCAF)”.
In the second category of assisted GPS mode of operation, the terrestrial network delivers to the GPS receiver almanac and ephemeris information that would otherwise be obtained from the GPS satellites at a low rate of 50 bits/second. The terrestrial network delivers the almanac and ephemeris record at a much higher data rate. The GPS receiver operates both a measurement engine (ME) as before and a position engine (PE). The position engine receives the pseudo-range information captured by the measurement engine and calculates the solutions to the GPS equations yielding true range, Doppler, time and position information for the given output of the measurement engine and the ephemeris tables for each visible satellite. This second category of assisted GPS is mobile terminal (MT) based. For the purposes of this disclosure, this second category of assisted GPS mode of operation may also be referred to as “device-computed assisted fix (DCAF)”. Also, as ephemeris tables become stale, the GPS receiver may request updated tables from the terrestrial network or may transition from DCAF to full autonomous mode (i.e., DAF).
Unfortunately, most GPS receiver chipsets operate in only one of the available modes: DAF, DCAF, or SCAF. For example, stand-alone GPS units from Magellan or Garmin are typically DAF only. Most of the E911 elements built into CDMA mobile phones operate only in SCAF mode. Many of the GPS units based on the SiRFSTAR model operate only in the DCAF mode.
However, newer GPS chipsets, such as the Texas Instruments TSL5001, can operate in any of the three modes. However, network configurations do not support a single device that operates in any of the three modes or that can shift dynamically between these modes on a single network. Moreover, while it is relatively easy to transition from the DCAF mode to the DAF mode on the TWL5001 chipsets, transitioning from SCAF to either DCAF or DAF is not straightforward.
Therefore, there is a need in the art for an improved GPS receiver that is able to switch between autonomous mode and assisted mode and terrestrial networks that support such mode switching.